Intro to Filters

Text Filters

One of the most common, and most useful, types of command line applications is a “filter.” A filter is any application which accepts text as its input, and provides the text, or a modified version of it, as its output. A simple example of this is the sort command.

Example file:

fruit.txt

bananas
kiwis
apples
tomatoes
oranges

> sort fruit.txt
apples
bananas
kiwis
oranges
tomatoes

Filters can do anything you can imagine, including modifying the output. Let’s see a couple examples of the grep command:

> grep e fruit.txt
apples
tomatoes
oranges

> grep an fruit.txt
bananas
oranges

In the first example, we look for any lines containing the letter e. In the second, we look for the letters an.

Filters can be used, among other things to:

search

Return only results matching search query.

reformat

Modify text. For example, but standardizing phone numbers from 5552121234 to (555) 212-1234.

replace

Replace abbreviations with full words, or even placeholder text with templates.